On s390x unsigned integer comparisons with immediates require the immediate
to be an unsigned 32-bit integer. The rule was checking that the immediate
was a signed 32-bit integer.

This CL also adds a test for comparisons that could be turned into compare
with immediate or equivalent instructions (depending on architecture and
optimizations applied).

Fixes #19940.

Change-Id: Ifd6aa989fd3d50e282f7d30fec9db462c28422b1
Reviewed-on: https://go-review.googlesource.com/40433
Run-TryBot: Michael Munday <munday@ca.ibm.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>